It seems that these days, running the emacs-snapshot package goes
quite smoothly.
I used to have the flavor troubles alluded to in the URL you cite, but
that has gone now. Now, packages seem to test for flavor != emacs
instead of flavor = emacs21. So packages get installed for
emacs-snapshot, too.
I specifically do NOT need to explicitly set the flavor.
One thing to make sure about is that you are actually running the
emacs-snapshot package, and not a self-compiled Emacs installed
elsewhere. Test with "which emacs".
Kai